A sigma (σ) bond is a covalent bond formed by end-on (axial) overlap of orbitals along the line joining the two nuclei. Because the electron density is symmetric about the bond axis, the bond permits free rotation of the two atoms about that axis. Every single bond is a sigma bond, and it is generally stronger than a pi bond.

σ 键是由轨道沿连接两核的直线作头碰头(轴向)重叠而形成的共价键。由于电子密度对键轴呈对称分布,该键允许两原子绕该轴自由旋转。每个单键都是 σ 键,而且 σ 键通常比 π 键强。